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36 MANAGINGCOLOUR - PART 1 Figure 2 Figure 3 DL, the difference on the green-red axis as Da and the differences on the blue-yellow axis as Db. The expression of colour variance as the difference in DL, Da and Db has resulted sometimes in good colour difference numbers, but a visual comparison of the colours has revealed that clearly the colours were definitely different. Another way to measure colour is the L*C*h*-plot. This plot looks quite similar to the L*a*b*-plot, but the axes are a little bit different. The L* axis is the Lightness, C* is the Chroma which defines the saturation of a colour, and h* is the hue which defines the colour (red, green, orange Figure 4 etc.). The hue is notated on a 360° circle, where red has an angle of 0°, yellow is 90°, green is 180° and blue is 270°. Since the hue defines the colour, differences in the hue angle are more visible than differences in chromaticity, or saturation. An L*C*h* plot is shown in Figure 3. In order to demonstrate the circular notation of Lightness, Chroma and Hue look at Figure 4 to the right. Hue and chroma (saturation) circle (courtesy of X-Rite) Screenshot of a colour standard measured at D50.10° with CMC-tolerance Screenshot of a colour measured with D50/2 and L*c*h*-tolerance As you can see, the center axis from bottom to top is the lightness of a colour. The distance from center defines the saturation of a colour, while a change in the angle will result in a different hue or colour family—for example, moving from blue to red. Since the hue angle has a major influence on the colour it is clearly understandable that a change in the hue angle is more easily noticeable then the change in the chromaticity of a colour. After explaining different ways of notating, or mapping, a colour, we need to have a look at establishing a tolerance around any given colour standard so we can compare two colours and determine if a measured sample is within the established tolerance level or not. Every colour has its unique L*a*b*-numbers and the difference between these colours is expressed by the term delta E, or DE, and a number behind it. There are many ways to establish colour tolerances or allowable differences around a colour standard. How this is done will be shown in future articles in this series. Be sure to read part 2 in the next edition of Canadian Printer. CP Dr. Martin Habekost is an Assistant Professor at Ryerson University’s School of Graphic Communications Management. He can be reached at (416) 979-5000 x 6951 or mhabekos@ryerson.ca www.canadianprinter.com APRIL/MAY, 2006
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